Constant power operation of a synchronous motor having magnets has been mentioned in the document: Proceedings of the IEEE, Vol. 70, No. 2, Feb. 1982, New York, US, pages 116-135; B.K. Bose "Adjustable Speed AC Drives--A Technology Status Review".
With a synchronous motor, in order to obtain a wide operating range at constant power, it is necessary to make use of a large amount of phase advance.
With a synchronous motor having conventional magnets, this large phase advance (up to 90.degree.) gives rise to too high a level of current when the motor is operating unloaded, which level may exceed the thermal limit for the parts in question and may present a severe risk of the rotor magnets becoming demagnetized, thereby reducing machine reliability and performance.
This solution also requires the use of an electronic power supply varying circuit having very high installed power compared with the mechanical power obtained, thus giving rise to a compromise which is economically unfavorable.
An object of the present invention is to provide a synchronous motor capable of operating at constant power over a wide range of speeds, e.g. lying between a given base speed N.sub.0 and four times said value.
Another object of the invention is to provide a control system associated with the above motor and suitable for attaining the following objectives:
1/ ensuring that under all possible situations the current is controlled and within permissible limits for the motor; PA1 2/ ensuring that the magnetization state of the rotor magnets is preserved, i.e. avoiding any risk of demagnetization; PA1 3/ obtaining a minimum value for motor iron losses during periods of operation at maximum speed; and PA1 4/ optimizing the ratio between the installed power of the electronic power supply varying circuit and the mechanical shaft power obtained.